i 



ASTRONOMICAL CIRCLE. §05 \ 



hand of the figure. It passes through an angle, In which it 

 rests in the same manner as the pivot of a transit instrument 

 does on its support. At the lower end of the tube which pro- 

 tects it, a smaller tube is fixed at right angles, which contains 

 microscopic glasses so contrived, that the image of a luminous 



point, like the disc of a planet, is formed on the plumb-line and ■ 



bisected by it. Great attention should be given to the accurate 1 



bisection of this transparent point by the plumb-line at ths nw 5 



mi:nt of observation. It is absolutely essential in instruments of •; 



this construction, to consider the observation, as consisting ia i 



two bisections at the same time: the one of the star by the mi- : 



crometer, the other of the plumb-line-point by the plumb-line. \ 



The least negligence in either of these bisections will render the \ 



observation unsuccessful. I 



The two strong pillars, which support the axis of the vertical Upright pillars i 

 circle, are firmly united at their bases to a cross barj to which and vertical - 

 also the long vertical axis is affixed, and which may be consi- ^owif la'stouc ' 

 dered as forming one piece with them. The stone pedestal is pedestal. 

 hollow, and contains a brass conical socket, firmly fastened to i 

 4he stone, and reaching almost to the ground. This socket re- 

 ceives the vertical axis, and supports the whole weight of the i 

 moveable part of the instrument, which revolves on an obtuse = 

 point of the bottom ; the upper part of this vertical axis is kept ' 

 steady in a right angle, having two springs opposite the points . 

 of contact, which press it against its bearings, and it th.us turns ■ 

 in these four points of contact with a very pleasant and steady ■ 

 motion. j 



The bar, in which the vertical axis is thus centered, is acted Adjustment of ! 

 on by two adjusting screws in directions at right angles, and ^^^ vertical 

 perfectly independent of each other. By these motions, the 



axis may be set as truly perpendicular, as by the usual method I 



of the tripod with feet screws, which could not in this case ; 

 have been employed. 



The frame to which this apparatus is attached, is fixed to '• 

 the corners of the hexagonal stone, by the conical tubes; l)e- 



tween which and the stone, the azimuth circle (which forms j 



one piece with the vertical axis) turns freely. The azimuth | 



circle of two feet diameter, consists of eight conical tubes, in- - j 



serted in the vertical axis, and which are united at their ends j 



by th© circular limb ; this is divided and read off exactly in a . 

 •imilar manner to ll^e other circle. 



A level i 



